1. Field of the Invention
The present invention generally relates to data storage devices such as tape drives and, more particularly, to a closed loop servo system that uses position error signals (PESs) generated by a tape drive to dynamically adjust a lateral and/or angular position of tape within the tape drive to enhance tape drive performance.
2. Relevant Background
Tape drives have been widely employed in industry for over thirty years due to their ability to store large amounts of data on a relatively small and inexpensive removable format. Typically, a removable cartridge holding a reel or spool of storage tape is initially loaded into a tape drive. After coupling the storage tape on the cartridge reel to a take-up reel of the tape drive (e.g., via respective leaders), the tape is unwound from the cartridge supply reel, moved past one or more tape head assemblies (e.g., each having one or more tape heads with one or more read/write elements/transducers in the case of magnetic tape) or optical pickup units (OPUs)(e.g., in the case of optical tape) for reading and/or writing of data, and wound onto the take-up reel via a drive motor. Next, the tape is unwound from the take-up reel, moved past the tape heads or OPUs, and wound onto the cartridge. The storage tape is uncoupled from the take-up reel prior to removing the cartridge from the tape drive.
Increased tape storage density is an endless goal that pervades tape design, tape drive design, and the like. Such increases have been achieved in a number of manners such as through the design and use of thinner tape substrates, various data compression techniques, increases in the number of data tracks extending along the tape, corresponding increases in the number of tape heads and/or data read/write elements (e.g., transducers) on each head, and/or the like. However, the number of parallel, longitudinal data tracks which can be established on tape media has been limited by the number of read/write elements which could be fabricated on a head to read/write narrower tracks. In this regard, tape drives have been designed to process a tape using a head having fewer sets of read/write elements than there are tracks on the tape. For instance, the tracks can be divided into groups, each group containing the same number of tracks as there are read/write elements on a particular tape head.
However, there is a practical limit to the ability of a tape head assembly to accurately and reliably record data to and read data from a tape having increasing numbers of increasingly narrow tracks. For instance, the various read/write elements of a tape head assembly may not always be able to precisely follow respective data tracks on the tape during read/write operations (resulting in read/write inaccuracies) due to factors such as tape edge variations, environmental thermal expansion and contractions, inaccuracies in the formatting of tracks on the tape itself, dimensional and spacing deviations during the manufacturing of the head, and/or the like. In this regard, tape drives include a tape head actuator that can rapidly adjust the position of the tape head assembly relative to the tape under servo control to precisely follow a set of tracks during read and write operations. More specifically, the tape head assembly includes servo read elements for reading servo information previously recorded onto one or more specially recorded servo tracks extending along the tape (e.g., parallel to the data tracks). The tape drive is configured to generate one or more Position Error Signals (PESs) (e.g., each of which provides a measure of a position of one of the servo read elements relative to one of the servo tracks on the tape) based on the servo information read by the servo read elements, where the PESs are employed by a position servo loop to determine the transverse position of the servo elements relative to the servo tracks. The loop then transmits a signal to the actuator to rapidly move the tape head assembly by very small amounts as necessary to enable following of the data tracks.
Another manner of increasing the degree to which the read/write elements of a tape head assembly can precisely follow respective data tracks during tape drive operation is via limiting lateral tape motion (LTM) of the tape in directions perpendicular to the longitudinally extending data tracks. Generally, LTM is the tendency of the tape to move laterally (e.g., in a direction generally perpendicular to the longitudinal direction of motion of the tape through the tape drive during read/write operations) due to poor quality tape media, excessive usage of a tape cartridge, excessive vibration or movement of the tape drive during operation, wobbling of the supply or take-up reel, imperfect tape path guides, and/or the like.
In this regard, LTM sensors mounted within the tape drive may be configured to sense an actual position of one edge of the tape relative to one or more tape path guides (e.g., rollers), a target tape path, and/or the like. When the difference between the actual position and an ideal position is greater than some threshold, corresponding signals may provide feedback to a closed loop control system or unit designed to automatically readjust the lateral position of the tape back to nominal. For instance, the control system or unit may utilize the feedback provided by the LTM sensors to adjust a position (e.g., height, tilt, etc.) of a tape path guide over which the tape travels to cause the tape to move laterally back into a desired position or range that increases the ability of the read/write elements of the tape head assembly to accurately follow their respective tracks (e.g., via the above-discussed processing of PESs).